There have been some different systems for mobile communication. One of them is a PHS (Personal Handy-phone System), which is called “second generation of wireless telephone system” and is standardized, for example, by ARIB STANDARD or RCR STD-28 V4.1.
U.S. Pat. No. 5,644,621 describes a radio telephone system, including a plurality of mobile stations (sub units) and a base station (main unit). According to this system, the base station is provided with a means for selecting one mobile station among the plurality of mobile stations. The selected mobile station is allowed to establish communication with the base station.
Japanese patent publication Kokai 2001-16642A describes a PHS system, in which transmission timing from a base station is automatically controlled to avoid signal conflict, for example, in FIG. 5. According to this system, a main unit supplies parameter data to cell stations (CS1-CSn) in synchronization with the same synchronization signal. Each cell station includes a control circuit, which generates a frame synchronization signal based on the parameter data. The frame synchronization signal is delayed from the synchronization signal.
Each cell station also includes a radio signal transmission circuit, which generates a pulse signal identifying a transmission timing of a control channel signal in accordance with the frame synchronization signal. A control circuit transmits a transmission detecting signal to the main unit. In response to a detection request signal from the main unit, the transmission detecting signal identifies a transmission timing of the pulse signal. The main unit compares transmission timing signals among all of the cell stations, and adjusts parameter data so as that transmission timings are prohibited from being in conflict among the cell stations.
Japanese Patent Publication Kokai No. 2002-152820 describes a PHS system, especially shown in FIG. 6, which provides opportunities of communication to mobile stations equally. A cell station (CS) has no available channel when data communication with two channels is performed with a mobile station (PS) 2-1. At that time, the cell station (CS) is not allowed to receive a link-channel establishing request from another mobile station (PS) 2-2. The cell station (CS) transmits a notification signal of refusal to the mobile station (PS) 2-2, while stores the rejected link-channel establishing request.
After data communication between the cell station (CS) and the mobile station (PS) 2-1, the cell station (CS) assigns a link channel to the other mobile station (PS) 2-2 in response to a link-channel establishing request transmitted from the mobile station (PS) 2-2. At that time, the other mobile station (PS) 2-1 is restricted to using one channel for communication.
In general, according to a PHS system, each of mobile stations searches the closest cell station for communication. Cell stations transmit control signals periodically, which are detected by mobile stations. In accordance with receipt of a control signal, a mobile station transmits a connection request signal to the cell station.
According to a conventional communication system, if enough cell stations are not provided, for example in a mountainous area, link-channel-requests might be conflicted among plural mobile stations. As a result, it would occasionally take a longer time to establish mobile communication (link channels).